The invention relates to surface-modified platelet-like pigments, i.e., platelet shaped, based on platelet-like substrates which are coated with a titanium dioxide layer and a thin alkaline earth metal titanate film on top of said layer of titanium dioxide.
Platelet-like substrates coated with titanium dioxide, such as, for example, mica platelets are often used for pigmenting plastics, cosmetics or coating compositions, such as, for example, paints or printing inks. However, a disadvantage is the low photostability of titanium dioxide, which, for example, in the case of white pigments, leads to a gradual greying. Upon incorporation of platelet-like substrates coated with titanium dioxide in a polymer matrix, surface reactions between the titanium dioxide surface and the surrounding polymer matrix are observed in many cases, which impair the aesthetic impression of the pigmented polymer. Thus, for example, reaction of the hydroxyl-containing titanium dioxide surface with phenolic stabilizer additives often leads to yellowing of the polymer; however, in addition there may be further impairing surface reactions.
The previous proposals for avoiding these problems frequently do not lead to satisfactory results or are only suitable for solving part of the problem. Thus, for example, silanization of the pigment surface proposed in DE 4,041,663 effectively suppresses surface reactions in many cases, whereas virtually no increase in photostability is observed. The application of further metal oxide layers is, inter alia also unsuitable, since, for example when colored oxide layers are used, such as, for example, iron oxide, the coloristic properties of the pigment underneath which is coated with titanium dioxide are altered completely and moreover the metal oxide layers then deposited on top often also have high surface reactivity.